Recently, substantial attention has been directed toward the development of lightweight tubular shafts, such as shafts for golf clubs, fishing poles, and/or pool cues manufactured from various composite materials, particularly from "pre-preg" material. Pre-preg composite sheets are formed by pre-impregnating fabric or strands of fiber, for example carbon or glass, within a binding matrix, such as epoxy resin. The binding matrix or resin is partially cured such that it holds the fibers together and forms a malleable sheet.
Shafts may be manufactured by wrapping in a predetermined way a set of plies of pre-preg composite sheet around a molding mandrel having an inflatable bladder thereon, placing the mandrel and plies in a mold having inner wall(s) of a desired shape and size, and heating the ply-wrapped mandrel to a predetermined temperature for a time sufficient to allow the resin comprising the plies of pre-preg composite sheet to completely cure. Generally, the molding mandrel used in such a process is a long rod having a hollow end and a plurality of holes communicating between the outer surface of the mandrel and the hollow end.
The inflatable bladder, typically of a latex material, is stretched over the mandrel prior to wrapping the mandrel with pre-preg material. The bladder covers the holes communicating the outer surface of the mandrel with its hollow end. The bladder covered mandrel is then wrapped with pre-preg material. While the ply-wrapped mandrel is cured in the mold (e.g. typically by heating), pressurized gas is introduced into the hollow end, the gas communicates through the holes and inflates the bladder to a predetermined pressure to force the plies of pre-preg against the inner wall(s) of the mold. After the shaft is cured, the mandrel and bladder are removed. The bladder is discarded, and the mandrel is cleaned and prepared to receive a new bladder for manufacturing another shaft. In some instances a bladder may be reused after its removal.
Alternatively, a mandrel without a hollow end and holes may be covered with a bladder and wrapped with pre-preg material. Before the ply-wrapped mandrel is inserted into the mold, the mandrel is removed and an air fitting is attached. The bladder and plies are then inserted into the mold and pressurized gas is then introduced directly into the bladder through the fitting, inflating the bladder and forcing the composite material against the inner wall(s) of the mold. The material is then cured and cooled, and the bladder removed.
The inflatable bladder is typically a prefabricated sleeve made using a dipping process. A dipping mandrel, typically a shaft of similar configuration to the molding mandrel but without a hollow end or holes, is dipped into material, such as latex. This coats the outer surface of the dipping mandrel with material, which is cured, forming a bladder. The bladder is stripped off of the dipping mandrel and is then ready to be stretched over a molding mandrel.
There are many drawbacks, however, associated with such conventional bladders. For example, the bladder is typically handled multiple times, adding unnecessary steps to the manufacturing process, reducing efficiency and increasing cost. In addition, excessive handling of the bladder increases risk that the bladder may be damaged, e.g., when it is removed from the dipping mandrel, and/or when it is placed on the molding mandrel.
Furthermore, the quality of the shafts produced may be compromised because conventional bladders increase the risk that the resulting shaft will have irregular composite densities or similar structural imperfections. This is due to the bladders, typically made from latex or similar material, having an elastic memory. When the bladder is placed onto the molding mandrel, the latex may be stretched unevenly, creating irregular localized stresses in the bladder. This may include uneven axial stretching along the length of the mandrel, uneven radial stretching along the surface of the mandrel, twisting and/or bunching of the bladder as it is placed on the mandrel or as it is handled after such placement. In addition, a single bladder configuration is frequently used on various molding mandrels which may have a smaller cross-section than the dipping mandrel used to make the bladder, resulting in an ill-fitting bladder. Thus, when the composite material is wrapped onto the mandrel, the bladder may consequently bunch or wrinkle.
Thereafter during the curing process when the bladder is inflated, the latex material will tend to return to a more uniformly stressed shape, such as by untwisting, moving axially or radially, or unfolding wrinkles. The fibers of the composite material will follow the movement of the bladder, changing the density of the fibers unpredictably, and creating a structurally inferior shaft.
Therefore, there is a need for a molding mandrel having a snug, uniformly fitted bladder which provides an improved quality shaft, by minimizing undesirable surface distortions or stresses in the bladder that may shift the fibers of the composite material.
In addition, there is a need for an apparatus and a process for manufacturing composite shafts which involve less handling and therefore provide a more efficient manufacturing process and less expensive finished products.